The present invention relates to optical imaging and, more particularly, to an apparatus and a method for synchronizing the velocity of an image of a moving object or target and the clocking of image sensor elements used to track the moving target.
A widely used method of electro-optical single-frame imaging involves mechanically sweeping an image past a linear array of photoelements and reading out the array once for each incremental advance of the image. In this way, nearly square frames or long strip-type frames of imagery may be obtained. Certain types of arrays, such as time delay and integrate (TDI) arrays operate the same way, differing primarily in that they collect signals over many line periods.
An early example of utilizing scanned linear imaging arrays is a device having a series of back-to-back silicon diodes in a structure made from two monolithic silicon diode arrays. The array was electronically scanned with the aid of a fixed voltage drop along one of these arrays, together with a voltage sawtooth applied at one terminal. Other types of electronically scanned linear arrays followed, including the use of charge-coupled device (CCD) arrays. The scanning function can be performed in excellent fashion with a simple CCD shift register coupled to the array of photodiodes.
In TDI imaging, each photoelement of the line-scan array is replaced by a light sensing CCD shift register. The TDI imaging CCD arrays are commonly constructed out of a plurality of closely spaced, parallel TDI CCD shift registers built on a photoelectric semiconductor substrate. The array of sensor elements or detectors are used to store an electrical signal representative of the time-integrated radiation intensity. An optical image is scanned across the surface of the photoelectric semiconductor substrate of the TDI imager along the length of the TDI shift registers. The shift registers are clocked in synchronism with the motion of the optical image. Electrons that are freed under one of the TDI shift registers by the photoelectric effect of light in a given portion of the optical image are collected into a charge packet that will be moved along by the TDI shift register in conjunction with that given portion of the optical image. When a charge packet reaches the end of its associated TDI shift register, it is fed into an output CCD shift register in parallel with all of the other charge packets which have reached the end of the their associated TDI shift registers at the same time. The output shift register rapidly shifts out all of the charge packets fed thereto, and provides a series of charge packets or pixel values. The variable charge levels of the series of charge packets correspond to the variable light intensity of a picture line taken from the two-dimensional optical image scanned across the TDI imaging CCD array of the TDI imager.
For a TDI imaging CCD array to function properly without image smearing, the charge packets must be shifted down the shift registers at the same velocity as the image being scanned across the surface of the CCD array.